Fiber reinforced composite materials composed of reinforcing fibers such as carbon fibers or glass fibers and a matrix resin such as an epoxy resin or phenol resin are lightweight and excellent in mechanical properties such as strength and stiffness, heat resistance and corrosion resistance, and therefore have been used in many fields such as aerospace, motor vehicles, rolling stock, ships, civil engineering & architecture, and sporting goods. Especially for applications requiring high performance, fiber reinforced composite materials using continuous reinforcing fibers are used; as reinforcing fibers, carbon fibers excellent in specific strength and specific modulus are popularly used, and as matrix resins, thermosetting resins, especially epoxy resins, which are excellent in adhesion to carbon fibers, are popularly used.
However, cured products of thermosetting resins such as epoxy resin are generally low in fracture toughness as compared with thermoplastic resins. This causes a problem that the impact resistance of the fiber reinforced composite material lowers. Especially in the case of structural members of aircrafts, since they are required to be excellent in impact resistance against such impacts as the drop of a tool during assembling and hail during flying, the enhancement of impact resistance has been a large issue.
Fiber reinforced composite materials generally have a layered structure, and if an impact is applied to it, a high stress acts between layers to cause cracking. For inhibiting cracking, it is effective to enhance the plastic deformability of an epoxy resin, and as a means for it, inclusion of a thermoplastic resin with excellent plastic deformability is available.
Such a technology, however, will result in forming a resin layer which serves as an insulating layer between layers whereas it will impart high impact resistance to fiber reinforced composite materials. Therefore, a disadvantage of that technology is that thickness-direction conductivity will lower, which is a kind of conductivity, which is a characteristic of fiber reinforced composite materials.
In recent years, great strides have been made in order to aim at achievement of both excellent impact resistance and excellent thickness-direction conductivity with a fiber reinforced composite material. For example, as disclosed in Patent Document 1 or 2, there is a method of disposing a reinforcement to enhance impact resistance and conductive particles to enhance thickness-direction conductivity in an interlayer section of a fiber reinforced composite material.